Direct injection engines are aimed at improving fuel economy at low engine loads by providing a stratified charge in the combustion chamber. A stratified charge engine is one in which the combustion chamber contains stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The overall air/fuel mixture within the combustion chamber is lean of stoichiometry, thereby improving overall fuel economy at low loads. At high engine loads, typically greater than 50% of full engine load, a homogeneous air-fuel mixture is provided in the combustion chamber.
Conventional direct injection engines typically include a piston having a depression in the top face thereof (typically referred to as a bowl) and a swirl or tumble control valve located in the intake port to produce a swirl or tumble of the air entering the combustion chamber. As fuel is injected into the combustion chamber, the fuel impinges against the bottom of the bowl and cooperates with the motion of the air in the chamber to produce the stratified charge, with the richest portion of the charge moving toward the ignition source.
The inventors of the present invention have recognized certain disadvantages with these prior art engines. For example, because the fuel sprayed from the fuel injector is directed toward the piston bowl, it is likely that a portion of the fuel will stick to the piston surface causing an undesirable wall-wetting condition. As the remainder of the fuel is burned, the flame propagating toward the piston surface is unable to completely burn the liquid fuel film on the piston surface. This results in undesirable soot formation during combustion.
In addition, because the design of these engines relies on the fuel impinging against the bowl and subsequently directed toward the spark plug, fuel injection timing is of a major concern. In direct injection engines, fuel injection is a function of time whereas the motion of the piston is a function of crank angle. In port injected engines, fuel entering the chamber is a function of crank angle because the opening of the intake valve is a function of crank angle. As a result, it is imperative to control the timing of fuel injection in a direct injection engine so that the injected fuel may impinge on the bowl at the proper time and the fuel cloud may move toward the spark plug. In other words, if the fuel is injected too early, the spray may miss the bowl entirely, thereby not deflecting toward the spark plug. If the fuel is injected too late, then excess wall-wetting may occur.
Further, the inventors of the present invention have found that with bowl-in-piston engines, switching between a stratified charge and a homogeneous charge occurs at part loads ranging between 30% to 40% of full engine load. As the engine load increases, more fuel is required. However, because of the physical limitations of the bowl (i.e. the size of the bowl relative to the size of the combustion chamber), the amount of fuel that can be placed in the bowl and still attain a stratified charge is limited. Otherwise, the potential for wall wetting and subsequent soot formation may increase. As a result, above about 40% of full engine load, fuel economy is compromised.
Other disadvantages with prior art engines results in a heavier piston, increased engine height to accommodate the larger piston, a larger combustion chamber surface to volume ratio, more heat loss, and increased charge heating during the intake and compression strokes, which increases the tendency for engine knocking.